Embodiments of the present invention relate to the treatment of water in an electrochemical apparatus comprising a water-splitting ion exchange membrane.
A water-treatment electrochemical apparatus comprises an ion exchange cell to treat water by selectively exchanging ions present in the water to remove contaminants, reduce total dissolved solids (TDS), treat industrial or hazardous waste water, desalinate salt water, and for other applications. The ion exchange cell comprises a water-splitting ion exchange membrane positioned between electrodes in a water-tight housing. When a current is applied to the electrodes by a cell power supply, water is irreversibly dissociated into H+ and OH− ions at the boundary between the cation and anion exchange layers of the membrane(s), causing cations and anions to be exchanged from the water stream passing through the cell. Advantageously, when the reverse electric potential is applied while flushing the cell with water, the membranes of the water-splitting ion exchange cell are regenerated without the use of hazardous chemicals or salt. For continuous operation, two or more ion exchange cells can be connected to allow treatment of water in one cell while another cell is being regenerated. The cell can also have a valve system to control the flow of water during treatment and regeneration processes. Such electrochemical ion exchange apparatus are described in commonly assigned U.S. Pat. Nos. 5,788,812; 7,344,629; 7,780,833; 7,959,780; 8,293,085; and 8,562,803; all of which are incorporated herein by reference in their entireties.
However, it was found that treatment of hard water in such electrochemical cells can cause failure of the apparatus after multiple treatment and regeneration cycles due to scale deposits. Hard water contains dissolved multivalent ions, such as for example calcium, magnesium or manganese ions, and bicarbonate or sulfate ions. During hard water treatment, these ions form compounds that precipitate out of the water being treated or when the cell is being regenerated, to form scale deposits on cell walls, water lines, valves and other components. For example, dissolved calcium and magnesium ions in the presence of bicarbonate ions can precipitate out in the form of calcium or magnesium carbonate compounds. The accumulation of scale on the walls and water lines can require frequent cleaning of these components. Scale accumulation can also increase the required water pressure by constricting water line openings and channels within the cell. Additionally, scale binding to the ion exchange membrane reduces its effective ion exchange surface area and the flow rate of water through the membrane causing poorer or slower deionization. Scale formation also results in valve and drain leaks, valve and drain clogging, and cell overheating.
For these and other reasons, further developments and improvements in scale reduction within electrochemical apparatus and their ion exchange cells are continuously being sought.